As is known, some internal combustion engines are equipped with a turbosupercharge system for increasing engine power using the heat content of the exhaust gas to compress the air intake of the engine and so increase volumetric efficiency.
A turbosupercharge system comprises a turbosupercharger with a turbine fitted along the exhaust pipe and rotated at high speed by the exhaust gas from the engine, and a compressor rotated by the turbine and located along the air intake pipe to compress the air intake of the engine.
In a turbosupercharge system, the operating range of the turbosupercharger must be kept within a useful area, depending on the engine operating condition, for both functional reasons (i.e. to avoid irregular or, at any rate, low-efficiency operation) and structural reasons (i.e. to avoid damaging the turbosupercharger).
More specifically, the useful operating range area is bounded, on the left of a reduced mass flow/compression ratio graph, by a pump line, and on the right by a so-called ‘saturation line’. The pump line thus defines a first ‘off-limit’ area, and is defined by the points in which the internal aerodynamic balance of the compressor is disrupted, with periodic, loud, violent flow rejection, and possibly destructive effects on the blades. The so-called ‘saturation line’ defines a second ‘off-limit’ area, corresponds to sonic conditions (and hence flow cutoff) at the turbine inlet, and defines the maximum flow the compressor can supply under given intake environment conditions.
Patent Application EP1741895A1 describes a method of controlling an internal combustion engine supercharged by a turbosupercharger comprising a compressor; a turbine for rotating the compressor using the exhaust gas from the engine; and a wastegate valve for regulating exhaust gas flow to the turbine, to control rotation of the turbine according to a target supercharge pressure from the compressor. The control method comprises the steps of measuring the air intake pressure of the compressor; determining mass flow of the compressor; calculating—by means of a compressor operation map, and as a function of the predetermined limit rotation speed, the air pressure measurement, and mass flow—a limit supercharge pressure correlated to the air pressure obtainable from the compressor when the turbine rotates at a speed substantially equal to the predetermined limit speed; determining whether a target supercharge pressure satisfies a given relationship with the calculated limit supercharge pressure; and activating the wastegate valve, if the relationship is satisfied, to control turbine rotation speed as a function of the limit supercharge pressure, and so rotate the turbosupercharger at substantially the predetermined limit speed.
Patent Application EP2014894A1 describes a turbosupercharged internal combustion engine control method, which comprises establishing, in a reduced mass flow/compression ratio graph, at least one limit operation curve; at least one intervention curve relative to a wastegate valve regulating a turbine bypass pipe; and at least one intervention curve relative to a Poff valve regulating a compressor bypass pipe. The control method described uses the limit operation curve to limit the engine control pressure target downstream from the compressor; opens the wastegate valve when the wastegate valve intervention curve is exceeded; and opens the Poff valve when the Poff valve intervention curve is exceeded.
The EP2014894A1 control method keeps the turbosupercharger within the useful area, regardless of the operating condition of the internal combustion engine, to prevent failure or damage, but fails to enhance performance of the turbosupercharger.
Patent Application EP1323912A1 describes a turbosupercharged internal combustion engine control method, which determines a parameter depending on mass flow of the compressor, and by which to control the turbosupercharger to protect the compressor and keep the actual pressure within the specified compressor operating range.
That is, the EP1323912A1 control method determines a parameter depending on the mass flow of the compressor, and by which to control the turbosupercharger to protect the compressor, keep the actual pressure within the safe operating range of the compressor, and so prevent failure or damage, but also fails to enhance performance of the turbosupercharger.